Shell and tube type heat exchangers customarily comprise a bundle of tubes mounted in spaced parallel relation at opposite ends in tube sheets. The tube bundle is surrounded by a shell through which a medium is flowed between an inlet and an outlet of the shell. Heads are provided on opposite ends of the shell to afford the flow of another medium through the interior of the tube bundles. In multiple pass heat exchangers, one head end of the heat exchanger may be provided with flow reversing means to cause the fluid flowed within selected tubes to make several passes before exiting the heat exchanger.
Heat exchangers of the above type have been known to fail in use and to require repair. Generally this is accomplished by removing the heads at opposite ends of the tube, and either removing and replacing the defective tube or plugging the defective tube. Since such heat exchangers are often integral components of continuous plant processes, it is important that they be repaired quickly with a minimum of down-time.
Various types of plugs are known for sealing off defective tubes. For instance, plugs marketed by Expando-Seal Tools, Inc. the Assignee of the present application, include a pair of rubber plugs which are expanded into tight engagement with the interior of the tube in the zone of the tube sheet. Such equipment is sold under the trade designation TBT Tools.
While the aforementioned TBT Tools function satisfactorily in many applications, they have certain limitations. For instance, the rubber stoppers limit the applicability of such tools to a certain temperature and pressure range and service life. In addition, the tools require that the actuating mechanism for the stoppers be left in place inside the tube bundle and head ends. Not only is this an uneconomic use of the actuator, but it also may effect adversely the flow of fluid in the head ends.
In U.S. Pat. No. 2,856,963, there is disclosed a plugging device for use in shell and tube type heat exchangers as described above. In the patented assembly, a two-piece plug is inserted in a defective tube in substantial alignment with the tube sheet and is connected by a breakaway connection to a pull rod which extends longitudinally inside the tube. The outer portion of the plug is restrained against axial movement of the tube by means of a series of fingers which engage spaced locations around the periphery of the tube adjacent the tube sheet. While the patented structure may function satisfactorily in certain applications, the proclivity for the fingers to straighten when substantial tension is applied to the pull rod limits the amount of tension which can be applied and hence the amount of sealing pressure. Moreover, since there is often welding present around the periphery of the tube where it engages the tube sheet, it is not always possible for the fingers to grip the tube properly with an even amount of stress applied to each finger.
While the plugging of heat exchanger tubes is difficult when the tubes are relatively free from scale and corrosion, it is even more difficult to provide a completely fluid-tight seal when the tubes have inner layers of scale and/or corrosion. In such cases, there may be a tendency for fluid to flow between the plug and the scale/corrosion covered tube. Moreover, it is important for plugs to be able to accommodate variations in manufacturing tolerances of heat exchanger tubes. Hence, a plug assembly which can handle all these conditions is particulary desirable.
A device for plugging holes in other applications is disclosed in U.S. Pat. No. 4,091,841.